Pulsed neutron irradiation is a known technique for logging a wellbore and the strata adjacent to the wellbore. Many type of annalysis can be obtained as a result of pulsed neutron logging.
A pulsed neutron generator tube obtained from Kaman Sciences Corporation of Colorado Springs, Colo. is installed in a sonde for use in downhole conditions. It serves as a source of pulsed neutrons for irradiation of the surrounding formations as the sonde traverses the wellbore. A pulsed neutron tube known as Model A-302A installed in a sonde required reasonably precisely controlled operating voltages and current. In the ordinary circumstance, a sonde is lowered on a multiconductor logging cable. Ordinarily, the AC power source for the system is maintained in the logging truck. The logging truck is normally parked on the surface, and the logging cable that is connected from the truck is spooled on a large storage drum to be fed into the wellbore. The cable is extremely long, and can even reach lengths as great as up to 30,000 feet. Of necessity, the cable must be longer than the depth of the well. The logging cable is ordinarily a multistrand cable encased within a sheath for protection.
The logging cable appears as a long line to an AC input, and therefore has an AC impedance interposed between the AC generator on the truck and the sonde which is supported by the cable. In part, this is aggravated as the pulsed neutron generator tube is switched from idle to 100% output. Such wide variations in current demand by the equipment in the sonde inevitably produces a wide fluctuation in input voltage at the sonde as a result of the impedance imposed between the sonde and the truck located generator. The variations are not minor; the variations can range as high as 40% in an extra long cable at full load conditions.
The present apparatus takes into account line voltage fluctuations in conjunction with the sequence of events required to power the pulsed neutron generator for logging operations. The sequence of events involves the supportive supply circuitry for the neutron generator tube.
This disclosure therefore sets forth an unobvious interlock system which enables stabilized operation of the neutron generator over a logging run under stabilized conditions. Consistency for its operation is assured by the disclosed apparatus. Moreover, the apparatus is able to test for and determine adequate terminal voltage at the sonde to power up the equipment, control the switch-on procedure, initiate a switch-off procedure as necessary and to also protect the neutron generator tube in the event of circuit malfunction. The procedure involved between switching on, switching off and protection in the event of a malfunction is implemented by the circuit means of this disclosure. It thus enables the apparatus to apply a relatively high voltage DC level to the target high voltage power supply driver circuit, also apply a 3,800 hertz, 6 microsecond pulse and a logic zero control signal to the same circuit. This increases the target high voltage power supply output to about 70 kilovolts. An additional operational step achieved on measuring the current drain to the high voltage power supply driver circuit is the formation of a 3,000 volt ion source pulse train and a replenisher current of about 2 amperes. As the pulled neutron generator tube emits neutrons which is indicated by measuring the target current at the low side of the target high voltage power supply, a logic one is applied to the control voltage for the target high voltage power supply driver circuit. This provides the target with the high voltage, nominally 100 kilovolts. In the event of malfunction, the ion source pulses are turned off. The replenisher current (approximately 2 amperes during full load) is reduced substantially to zero, and the target high voltage is reduced from about 100 kilovolts to about 70 kilovolts. These protective steps prevent damage in the event of loss of adequate terminal voltage at the sonde.
One advantage of the present apparatus is the feature which checks terminal voltage at the sonde to verify that the proper voltage level has been furnished. In the event the voltage level is improper, the equipment does not initiate operation of the neutron generator. Rather, it will not start and thereby avoids damage, namely the risk of damage to the expensive tube. While neutron generator tubes can be replaced, it is not an easy procedure to carry out in the field. The neutron generator tube is a relatively expensive device and is protected.
The present invention may therefore be summarized as a pulsed neutron generator tube control circuit capable of monitoring the line voltage applied to it for initiating the turn-on sequence of the components for operation of the generator tube. The apparatus includes an interlock arrangement whereby successful turn-on of the various signals is observed to carry out the various steps providing adequate power to the pulsed neutron generator tube. Moreover, the equipment controls a procedure for switching off, the procedure having a sequence which prevents damage to the generator tube.